The present invention relates to a method and apparatus for stretching and smoothing the laser pulses of a ArF and F2-excimer lasers by using optical delay lines. The optical delay lines allow the shaping and tuning of the temporal beam profile of the laser pulses. More particularly, the present invention provides optical delay lines which provide multiple parallel beam paths and a plurality of locations for output coupling.
Deep UV excimer lasers with wavelengths below 200 nm and high pulse energies of several mJ are applied in the photolithography resist exposure process used in the production of integrated circuits. For photolithography imaging very expensive and large optical lenses are used. The shorter the wavelength, the greater is the problem of photoinduced damage of the employed optical components. Color center formation and compaction effects are examples of such damage. The occurrence of optical material damage depends nonlinearly on the peak intensity of the applied laser pulses. If it is possible to stretch the pulse by a factor of 2 or greater, thereby reducing the peak intensity of the pulse, the lifetime of optical components can be increased by almost one order of magnitude. In other words, the longer the duration of the light pulse and the lower the peak beam intensity, the longer the lifetime of all optical components in the microlithography setup.
Today, the only light source in the deep UV spectral range with sufficient power for industrial applications are excimer lasers. Due to the highly complex electrical discharge process needed for the generation of light emitting excimer states, the laser pulse duration has an upper limit of a few tens of nanoseconds. At present, pulse durations of about 30 ns can be achieved; in the future pulse durations of about 50 ns should be possible by appropriate modification of electrical discharge circuits. The disadvantages of pulse stretching methods using electrical rather than optical methods are that other important laser beam parameters such as pulse-to-pulse stability or pointing stability are strongly influenced as well. Furthermore, electrical pulse stretching can be optimized usually for one special type of laser only, whereas optical pulse stretching is completely independent of the given laser configuration.
The Koybashi et al. patent (JP 6-214187) discloses a prior art device which functioned as an optical delay line or circuit for a pulsed laser beam. FIG. 1 is an example of this prior art delay circuit. A laser beam (1) exciting the laser encounters a flat partially reflective mirror (2) which splits the incident laser beam into a portion (1A) which is transmitted as an output beam and into a portion (1B) which is to be delayed. The portion to be delayed (1B) is reflected perpendicularly toward a circuit of four flat reflective mirrors (3) whose reflective surfaces are so arranged as to return the reflected portion of the incident light beam to the partially reflective mirror (2). These mirrors comprise an optical delay circuit. The circuit delays the transmission of incident light by providing a longer path for light conduction. Upon reencountering the partially reflective mirror (2), the delayed portion is split once again. However, this time it is the reflected light portion which exits the optical delay device. This exiting light is delayed by an amount of time required to travel through the light pathway defined by the four reflective mirrors. The delayed portion which is transmitted through the partially reflective mirror is iteratively subjected to the above process. This extra and variable time required to traverse the circuit stretches the pulse out temporally. Light transmitted through the partially reflective mirror (2) was subject to refraction (not shown). One limitation of this prior art device was therefore the effect of refraction on the optical spread of the laser beam.
The Kobayshi et al. patent introduced several improvements to the above system which were focused upon stabilizing the optical axis of a processed laser beam of no specified wavelength and without apparent respect to any particular industrial application. One improvement was the use of an optical fiber, rather than a system of mirrors, to delay the transmission of a portion of an incident light beam. However, while this approach eliminates the problems due to refraction and the need to keep reflective surfaces tightly aligned, an important limitation was the substantial transmission losses of light energy by passage through the optical fiber. Such losses especially limit the applicabiity of this design to UV and DUV laser light.
Another improvement introduced by Kobayashi et al. was to add a compensation plate to partially correct for the effect of refraction on output beam divergence. Kobayashi also evacuated the optical delay circuit to eliminate the absorption of the light by air and the influence of air on the alignment of the reflective surfaces. FIG. 2 shows an optical delay circuit which operated within an evacuated chamber (not shown) and employed prisms (4) as the internal reflective means. A compensation plate (5) was introduced to correct the effects of refraction on the light beam as it passed through the partially reflective plate (2). One limitation of this compensation plate arrangement was that light beams which underwent multiple refractions upon passage through multiple loops of the optical delay circuit, were corrected for only a single refraction event, and not the additional refractions involved in their multiple or iterative passages through the circuit. In another improvement, Kobayashi et al. addressed this limitation by substituting a cubic beam splitter (not shown) for the partially reflective mirror (2). The cubic beam splitter presented a perpendicular face to each incident laser beam to avoid refraction of the incident beams and the effects of refraction on the optical axis.
The design of the above optical delay lines limits their ability to stretch out a laser pulse and smooth the temporal profile of the optical pulse. For instance, where the laser light is half-reflected and half-transmitted upon each passage through the beam splitter, one-half the initial beam exits relatively unimpeded, one-quarter exits impeded by one circuit of the delay path, and only one quarter of the incident light is impeded by more than one passage through the optical delay circuit. A very small proportion of the incident light therefore is delayed by iterated passages through the optical delay circuit; and as much as half is emitted unaffected by the optical delay methods. Thus, the peak light intensity is not as broadly distributed and reduced as necessary to satisfactorily reduce the damage associated with the peak light intensity.
There is thus a need for a novel designs of optical delay devices for stretching and smoothing the laser pulses of ArF- and F2-excimer laser for microlithography applications where typical pulse durations fall in the nanosecond range (typically 20 to 50 ns).
The object of the invention is to stretch the laser pulse of ArF- and F2-excimer lasers using various improved designs of optical delay lines, including, if necessary, imaging optics. To provide stable optics, the delay line comprises a minimum of optical components with high reflecting coatings and uses as much as possible total internal reflection for beam direction changes. Furthermore, by using delay lines with multiple points of output coupling and multiple, parallel beam pathways, it is possible to achieve an even smoother shape and greater stretching of the temporal pulse profile. By this method it is possible to greatly smooth the temporal pulse envelope in order to avoid peaks with high intensity.
By using optical pulse stretching, the lifetime of optical components, especially for microlithography applications, can be increased to up to one order of magnitude.
By this invention the already known principle of optical pulse stretching by employing optical delay lines is further developed for deep UV (DUV) ArF- and F2-excimer lasers with a pulse length of a few tens of nanoseconds. The invention contains methods to achieve very high pulse energy transmission even in the deep UV. Using this technique, stretching factors  greater than 2 can be easily achieved.
The instant invention relates to a method for stretching of laser pulses of ArF- and F2-excimer laser for microlithography applications with typical pulse durations in the nanosecond range (typically 20 to 50 ns).
This special layout of delay lines can also be used for applications in the visible and infrared spectral range (e.g., with CO2 or Nd-YAG laser) for any application where pulse stretching is required in order to fit the industrial process conditions.
One could also use other beam splitters or beam combiners, for example methods, which make use of different polarizations of the direct beam and the delayed beam.